The earliest and simplest type of a capacitive pressure transducer, in which diaphragm deflection as a function of fluid pressure causes a variation in the distance between a pair of surfaces which form the plates of a variable capacitor, is illustrated in U.S. Pat. No. 3,634,727. Therein, a capacitor is formed of two wafers of silicon insulated from each other by glass and joined together by a low temperature glass or by brazing thin metal films deposited on the glass. In U.S. Pat. No. 4,405,970, there is described capacitive pressure transducers, manufactured with microcircuit technology so as to produce a large number of transducers by processing of a pair of silicon wafers. Therein, a pair of wafers (one or both of which have been processed to provide proper materials, shaping and surfaces for the formation of pressure transducers), are joined together by field assisted vacuum bonding. In addition, the non-pressure-responsive (parasitic) capacitance is reduced by the utilization of borosilicate glass wall structures which separate the fixed portions of the two capacitors a long distance from each other, whereby the fixed capacitance is reduced. However, as described therein, there is a limit as to the extent of borosilicate glass which can be utilized with silicon due to the variation in temperature coefficient of expansion which can result in cracking, and the like, if an excessive amount of borosilicate glass is used. The use of borosilicate glass as in said '970 patent also provides the mechanism for controlling the spacing of the capacitive plates, rather than relying on the length of time of etching to control the spacing of the plates.
In U.S. Pat. No. 4,415,948, a layer of borosilicate glass is applied to one of the plates of the capacitor to avoid arcing during field assisted vacuum bonding. However, the glass, occupying space between the plates of the variable capacitor, reduces the range of relative motion between the two plates, and lowers the sensitivity of the device. A commonly owned, copending U.S. patent application entitled "Three Plate, Silicon-Glass-Silicon Capacitive Pressure Transducer", Ser. No. 527,531, filed on Aug. 29, 1983 by Grantham and Swindal and now U.S. Pat. No. 4,467,394 issued Aug. 21, 1984, discloses the use of a third conductive layer, thereby essentially providing parasitic capacitance in series, which reduces its effective capacitance to a small level.